This invention relates to a low noise, high thermal stability attenuator of the integratable type, particualrly for attaining volume control over the audio signal through variable attenuators.
In order to attain volume control over the audio signal, two different techniques have been used heretofore. In particular, for TV applications, audio processor control devices are employed which operate on purely analog techniques, such as variable gain amplifiers. However, such devices exhibit problems relating to their temperature stability.
Another class of volume control devices comprise step-wise variable attenuators with attenuator elements connected in structures of the ladder type through switch systems which, in accordance with a control signal, provide electric connection and disconnection of such attenuator elements. Physically, such attenuators are formed by either switched resistors or switched capacitors in CMOS technology. In practice, such networks form bipolar digital-to-analog converters accepting at their inputs both positive and negative signals. In these devices, the digital word which controls the device is input connected, and the analog input is sent to the non-switched part of the network. These systems afford definitely good results and are highly advantageous in respect of their temperature stability and interfacing with audio processor systems. However, being implemented in the CMOS technique, they exhibit a high noise, especially at low frequencies, and require a fairly complicated process for the formation of CMOS-compatible resistive networks.